Manifold tuning

ABSTRACT

The invention relates to a manifold system for an internal combustion engine comprising a shutter (22) serving as an externally controlled source of pressure waves disposed within a tract (10) and activated at a predetermined time preceding the closing of an associated engine valve (12) such that the pressure pulse from the shutter reaches the associated valve (12) as it is closing during each engine cycle.

The present invention relates to the tuning of inlet and exhaustmanifold systems of internal combustion engines.

It is well known that some degree of supercharging can be achieved byselecting the length of the intake system to take advantage of thepressure waves caused by the rapid opening of the intake valve, whichpressure wave propagates back and forth along the intake tract in aseries of compression and rarefaction waves. By choosing the tractlength such that the time taken for the compression wave to arrive backat the intake valve is approximately the same as the time interval ofthe intake valve opening period, the density of the intake charge as itis being trapped inside the engine cylinder can be momentarily increasedthereby increasing the power output of the engine.

As the speed of sound is substantially constant (approx. 350 m/s inair), the time taken for the pressure wave to traverse an intake tractof fixed length for a fixed number of reflections is also constant. Onthe other hand, the duration of the intake event, if constant whenexpressed in terms of crankshaft angle, will vary inversely with enginespeed. Thus, the time from the start of the pressure wave perturbationto the instant when the intake valve closes is not constant andprogressively shortens with increased engine speed. It will be clearfrom this that for a fixed length of intake tract, there is only onespeed at which the desired matching or tuning of the intake tract isachieved. The resulting torque curve for such an engine would exhibit anarrow high peak at the tuned engine speed and the torque would fall offrapidly on each side of this peak.

Variably tuned intake systems have attempted to achieve increased torqueover a broader speed range by providing a control enabling the length ofthe intake tract to be varied in dependence upon engine speed in orderto match the changing time delay requirements.

The tuning of engine manifolds to take advantage of pressure waves isnot restricted to the intake system but can also be used in exhaustsystems. In the case of exhaust manifolds, tuning is used to achieveimproved scavenging, that is to say removal of residual exhaust gasesfrom the combustion chamber. Here, a negative pulse is required at theexhaust valve as it is being closed.

Manifold tuning can furthermore be employed not only to increase enginepower output but to derate an engine under part load operation. Thus, itis possible in the case of an intake system to arrange for a negativepulse to be present during intake valve closing as this reduces the massof air without throttling, thereby reducing pumping losses.

Both the fixed length and variable length manifold systems previouslyproposed are passive systems in as much as the pressure waveperturbation used to alter the charge density is created by the engineitself and the various designs of the manifold systems have only beenintended to optimise the phasing of a process which occurs naturallywithin the engine.

U.S. Pat. No. 3,254,484 proposes the use of acoustical resonators andsound generators in intake and exhaust systems to achieve effectssimilar to manifold tuning. However, this patent does not identify asound generator which can produce sound waves having sufficient energyto alter the intake charge significantly and relies insteadpredominantly on acoustical resonance.

The present invention does share with the tuned manifold systemdescribed above the fact that a shock or sound wave is employed to varycharge density at the instant of valve closing but seeks to providegreater control over the process so as to enable regulation of thebreathing efficiency to be achieved over a wide range of engine speeds.

In other known systems, such as EP-A-0194503, GB-437321 and EP-A-0141165a secondary valve is placed in series with the inlet valve and remainsclosed for some length of time after the inlet valve has opened. Theeffect of this is to develop a drop in pressure in the cylinder, whichstarts a surge of air flow when the secondary valve opens. The negativepressure wave created by this surge travels the length of the manifoldaway from the cylinder and is reflected as a positive pressure wave toincrease the cylinder pressure at the instant of closing of the maininlet valve.

The time of opening of the secondary valve is important if the positivepressure wave is to reach the main inlet valve as it is closing. Thistiming must be varied as a function of engine speed and at some speedsthe intake process may not commence until long after top dead centre(TDC). The effect of this is two fold. First, the piston must do work todevelop a vacuum and this reduces the available output power. Secondly,the shortening of the effective air intake event reduces the steady flowvolumetric efficiency and this counteracts the benefit of the positivepressure wave at the instant of closing of the main inlet valve.

In U.S. Pat. No. 4,691,670 a valve with variable opening and closingtimes is arranged in series with the main inlet valve. The valve isinitially closed when the main valve is opened to create a vacuum in theinlet manifold. When the secondary valve opens, the air column startsmoving with a higher velocity than would occur with normal induction andby correctly timing the opening one can ensure that the column of airreaches the main inlet valve with high velocity as the latter is closingand increase the charge density in the cylinder by the ram effect of themoving air column. The variable closing time is intended to prevent backflow from the cylinder. This system relies on the inertia of a movingcolumn of air and not on the pressure wave associated with thepropagation of a sound wave. Pumping losses also occur as the pistontries to generate a vacuum in the inlet manifold, but such losses arenot significant when the engine is operating under full load.

According to the present invention, there is provided an internalcombustion engine having a plurality of cylinders each with a camshaftoperated main inlet and main exhaust valve, and a manifold system whichcomprises a plurality of tracts each leading to a main valve of arespective cylinder and a plurality of secondary valves each disposed ata distance along one of the tracts in series with a respective mainvalve and arranged to open and close at an engine speed dependent phaseangle relative to the valve camshaft during each engine cycle so as tocreate a pressure wave which propagates at the speed of sound at leastonce along the length of the tract and reaches the associated main valveas the latter valve is closing, characterised in that each secondaryvalve is open at the instant of opening of the associated main valve andis operative, while the associated main valve remains open, to interruptfor a predetermined cranking angle the flow of the column of air in thesection of the tract between the secondary valve and the associated mainvalve, thereby creating the desired pressure wave, the latterpropagating initially in the direction of the main valve.

As compared with the above prior art references, for exampleEP-A-0194503, the invention offers the advantage that the duration ofobstruction of the manifold while the main valve is open is fixed incranking angle and as a result there is no variation in event durationwith engine speed tending to reduce the steady flow volumetricefficiency. The benefit of pressure wave tuning can thus be obtainedover a larger range of engine speeds. Also, because only a briefinterruption is required, there are no significant pumping lossessuffered by the piston.

An important advantage of the use of an active system of pressure wavegeneration is that one is no longer tied to tuned lengths and one caninstead vary the timing of the pressure pulse to suit the existingmanifold tract length. Previously, to achieve tuning at low enginespeeds required long tracts which were difficult to package within theengine compartment of road vehicles and the technique of intake manifoldtuning was used mostly on racing cars where the tuned manifold lengthsare shorter because of the higher engine speeds.

The most common requirement for such an intake system is to increase thevolumetric efficiency under full load and the system will therefore mostfrequently be employed in the intake manifold to produce a positivepressure pulse for each intake valve as it is closing. It will be clear,however, that the system can equally be used to derate the engine underpart load by transmission of a negative pressure pulse and can be usedin an exhaust system to improve scavenging.

An advantage of the secondary valve used in the present invention togenerate a pressure wave is that the power in the pulse is derived fromthe air flow itself rather than by the pumping action of the piston.Indeed the presence of the dynamic tuning system is hardly evident tothe piston at the instant the shutter closes, the only differencenoticeable being a brief surge of pressure caused by the arrival of thepressure wave.

Shuttering of fluid streams is, of course, well known as a source ofpressure pulses, this being well demonstrated by the effect of waterhammer.

The secondary valves, which may be constructed as shutters, should bearranged to send a wave to the associated cylinders but to no othercylinder. Thus, in the case of a fuel injection system with individualintake tracts connected to a plenum chamber, one shutter for eachcylinder may be arranged within the plenum chamber facing the associatedthe tract without restricting the air flow into it.

Because of the total enclosure of the shutters within the intakemanifold system, noise nuisance may be negligible but where the soundemitted is objectionable then it may be counteracted by a source ofanti-sound as the source of the sound is localised.

The activation of the shutters must be correctly timed relative to thevalve closure to achieve the desired effect. The mechanism required forachieving this aim is no different in principle from the mechanismneeded for achieving spark timing.

It is preferred that the shutters should be rotary shutters consistingof a part-cylindrical vane rotating in a cylindrical cavity and servingperiodically to close the tract leading to the associated valve. Avariable phase mechanism may couple the rotary vane for rotation at thesame speed as but with variable phase relative to the camshaft.

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic section through an embodiment of the presentinvention, and

FIG. 2 is a diagram showing the opening angles of the main valve and theshutter.

A tract 10 of an intake manifold leads from the main butterfly valve 14which controls the engine speed to the intake port 12 of one of theengine cylinders. The tract includes a cylindrical chamber 20 withinwhich there is arranged a rotary shutter 22 having a part-cylindricalvane 24. As the shutter 22 rotates, the air flow down the tract isinterrupted when the vane 24 blocks the tract 10 but is unrestrictedotherwise.

As the pressure pulses in this case are of the wrong sign to increaseengine performance, they are timed to reach the valve port 12 afterreflection at an open end of the tract to cause it to be inverted.

The timing of the opening of the valve and the shutter 22 are shown bythe timing diagram of FIG. 2. It will be noted that when the main valvestarts to open, the shutter 22 is already fully open and there is noincreased pumping by the piston in the first part of its down stroke.The shutter is closed briefly while the valve is open and the fullduration of the obstruction by the shutter is determined by thecircumferential length of the vane on the shutter and is therefore offixed cranking angle. Because of the separation of the shutter 11 fromthe valve, the moving air column between them will act as a weak springmaking the obstruction almost imperceptible by the piston. The kineticenergy of part of the air flow is nevertheless converted by theinterruption into potential energy stored in a pressure wave whichpropagates at the speed of sound towards the valve and eventuallyproduces the desired pressure wave tuning of the manifold.

The air flow and fuel metering are controlled in a conventional manner.The throttling of the air flow and the fuel metering may be carried outupstream or downstream of each shutter.

If increased power output is required, it is arranged for a positivepressure pulse to compress the charge while the inlet valve is closing.The wave propagating from the shutter takes a finite predetermined timeto reach the inlet valve. The shutter must therefore be activated afixed time before inlet valve closure and the control for the shutter isachieved in the same manner as the control of the spark timing. In theillustrated embodiment, the shutter 22 is driven by a belt 26 from theengine camshaft and a variable phase mechanism is included in thetransmission train to vary the phase of operation with engine sped. Thevariable phase mechanism may be externally controlled or it may employcentrifugal fly weights.

In a conventional or passive tuned manifold, it is believed that theopening of the inlet valve causes transmission of a positive pressure orcompression pulse towards the open end of the tract. This pulse isinverted and reflected a the open end so that a negative pressure orrarefaction pulse is sent back to the valve. This negative pressurepulse is reflected without inversion from the closed combustion chamberback towards the open end and after a further reflection with inversionagainst reaches the inlet valve as a compression pulse to compress thecharge as the valve is closing. The pressure pulse is thus attenuated bypassing four times up and down the intake tract and by three imperfectreflections at the open and closed ends of the tract.

In the present invention, by contrast, the active generation of thepressure pulse enables a more powerful and better controlled pressurepulse to be created, and the pulse need not be attenuated by longpropagation paths and multiple reflections. The extent to which thecharge density can be modified may therefore be potentially greater thancan be achieved by passive tuning and the control can be made to extendover a broad range of engine speeds without any alteration to the lengthof the intake manifold and without creating the packaging problems oflarge passive tuned manifolds.

In the case of a fuel injected engine, as illustrated, the chambers 20of the different cylinders may be joined to form the plenum chamberconnected by way of a common throttle 14 to an air filter box. Thetracts 10 in this case are the ducts leading from the plenum chamber tothe individual engine cylinders.

It is possible to provide an external source of anti-sound if the noisegenerated by the shutter 24 is found to be objectionable. Because thetiming of the noise source is predictable, almost complete cancellationof the sound can be achieved by suitable control and positioning of thesource of anti-sound.

The invention has been described above by reference to a mechanicalshutter as the secondary valve. It is alternatively possible to use afluidic device as a throttling device in place of a mechanical shutter.Such fluidic devices are known per se and are sometimes referred to asvortex amplifiers. In a typical vortex amplifier, a disc like chamberhas an axial exit port, a radially directed intake port and atangentially directed control port. In the absence of a flow into thecontrol port, fluid flow follows a short generally radial path from theintake port to the exit port and encounters little resistance. With asmall control flow, the main flow is forced into a swirl or vortex andencounters much greater resistance on account of the increased pathlength from the intake port to the exit port. Thus, such a device actsas a secondary valve controlled by the supply of fluid to its controlport.

The advantage of such an embodiment is that the timing of the secondaryvalve can be controlled at will using solenoid valves to switch thecontrol flow on and off and has no mechanical parts to wear out or causefriction. Furthermore, the interruption can be more sudden and brieferthan when using a mechanical shutter because of the limitations imposedby the physical size of a mechanical shutter blade.

It has been found experimentally, using a mechanical shutter for thesecondary valve, that an improvement of some 10% in the volumetricefficiency can be achieved by correctly timing the shuttering as afunction of engine speed. With a tract length of 700 mm, the bestimprovement is achieved at 2000 r.p.m. with the shutter closing at 75°ATDC during the intake stroke. The improvement in volumetric efficiencyis a function of both timing and the intensity of the wave generated. Itis important that the timing should correspond to the arrival time ofthe positive pressure pulse at the inlet valve while it is closing andthis determines the timing, that is the crank angle, uniquely for anygiven engine speed. The reason that the improvement in efficiency varieswith engine speed is that the energy in the moving air column varieswith crank angle and the energy in the sound pulse created by shutteringthe moving column will change with the crank angle at which the shuttercloses. Thus at engine speeds other than 2000 r.p.m., in the examplegiven above, the improvement that can be achieved is only a fraction ofthe optimum value.

The speed at which the improvement is optimised is dependent upon thetract length and by altering the tract length one can maximise theimprovement in volumetric efficiency at different engine speeds.

I claim:
 1. An internal combustion engine having a plurality ofcylinders each with a camshaft operated main inlet and main exhaustvalve, and a manifold system which comprises a plurality of tracts eachleading to a main valve of a respective cylinder and a plurality ofsecondary valves each disposed at a distance along one of the tracts inseries with a respective main valve and arranged to open and close at anengine speed dependent phase angle relative to the valve camshaft duringeach engine cycle so as to create a pressure wave which propagates atthe speed of sound at least once along the length of the tract andreaches the associated main valve as the latter valve is closing,characterised in that each secondary valve is open at the instant ofopening of the associated main valve and is operative, while theassociated main valve remains open, to interrupt for a predeterminedcranking angle the flow of the column of air in the section of the tractbetween the secondary valve and the associated main valve, therebycreating the desired pressure wave, the latter propagating initially inthe direction of the main valve.
 2. An engine as claimed in claim 1,wherein the manifold system an intake system and the secondary valve isa shutter activated to produce a negative pressure pulse which reachesthe associated intake valve as it is closing as a positive pressurepulse, having undergone at least two reflections at the ends of themanifold, the pressure wave serving to increase the density of thetrapped charge and thereby increase engine performance.
 3. An engine asclaimed in claim 1, wherein the manifold system is an intake system andthe secondary valve is a shutter activated to produce a pulse whichreaches the associated intake valve as it is closing as a negativepressure pulse, whereby to reduce the density of the trapped charge andthereby derate engine performance.
 4. An engine as claimed in claim 1,wherein the manifold system is an exhaust system and the secondary valveis a shutter activated to produce a pulse which reaches the associatedexhaust valve as it is closing as a negative pressure pulse, whereby toimprove scavenging of residual exhaust gases.
 5. An engine as claimed inclaim 1, wherein the secondary valve is a rotary shutter consisting of apart-cylindrical vane rotating in a cylindrical cavity and servingperiodically to close the tract leading to the associated main valve. 6.An engine as claimed in claim 5, wherein a variable phase mechanismserves to couple the rotary vane for rotation with the camshaft but witha variable phase relative to the camshaft.